The term “directional coupler” refers in general to a four-port, or a three-port when internally terminated, passive microwave device, where a main line conductor (also called the “through” line) carries radio frequency (RF) power. The main line conductor is in close proximity and is coupled to a secondary conductor by the electromagnetic field generated by the RF signal. The RF current flowing forward through the main line will induce RF current flow in the coupled conductor flowing in the opposite direction, and will only appear at one of the coupled ports (i.e., a signal current flowing from left to right on the main line will induce a signal current flowing from right to left in the coupled conductor and appear only from the left coupled output). As a result, the coupled output of forward and reverse flow of RF current, on the main line, will appear at different coupled outputs.
Many different designs for directional couplers, including both single and dual directional couplers, have been historically known, for example, from U.S. Pat. Nos. 6,066,994, 6,573,807 and 6,600,307. However, the directional couplers disclosed in these references suffer from a number of disadvantages, particularly in their design and their manufacturability.
U.S. Pat. No. 7,429,903 represented a significant advancement over the theretofore known designs, in providing an airline directional coupler that exhibited relatively high power, relatively low passive intermodulation (PIM), relatively low loss, and relatively wide bandwidth, all in a package that was relatively simple in design and relatively inexpensive and easy to manufacture. Though U.S. Pat. No. 7,429,903 was specifically directed to a dual directional coupler, the teachings thereof would be applicable to a single directional coupler.
However, while U.S. Pat. No. 7,429,903 was indeed an advancement over the prior art thereto, the teachings thereof still suffer from significant disadvantages. For example, although the frequency bandwidth of the design disclosed in U.S. Pat. No. 7,429,903 was relatively wide, as compared to historical designs, it still resulted in limitations. More specifically, the stepped configuration taught be U.S. Pat. No. 7,429,903 is frequency limited by the number of ¼ wave sections or ‘n-sections’, and is not practical to implement for a large number of sections.
What is desired, therefore, is a high power, low passive intermodulation (PIM), low loss, wide bandwidth airline directional coupler that is relatively simple in design and relatively inexpensive and easy to manufacture as compared to known designs, while at the same time also providing for wider bandwidth than has heretofore known designs.